Thermoplastics
A number of methods are known for improving the toughness properties of thermoplastics. These include incorporating impact modifying polymers, such as methacrylate-butadiene-styrene resins, which are multi-stage polymers having a butadiene polymer or copolymer, optionally containing vinylaromatics, as for examples styrenics, (meth)acrylic and methacrylic copolymers, into thermoplastics such as polycarbonate resins or polyester-polycarbonate blends; these copolymers typically possess a core-shell (multi-stage) morphology.
Unfortunately, the aging behavior of these impact modifiying modifiers at elevated temperatures is not satisfactory, and this limits their application, for example for automotive parts. The object of the present invention was, therefore, to provide a toughened thermoplastic composition which, compared with known systems, possesses, at temperatures down to -40.degree. C., considerably improved toughness properties, and at temperatures up to 160.degree. C. good aging properties combined with a satisfactory thermostability in the melt at temperatures up to 320.degree. C.
It has now been found that this object can be achieved for thermoplastics, surprisingly, by employing a specially stabilized toughening component, namely a stabilized core shell rubber-elastic graft copolymer composed of a rubber-elastic polybutadiene core, a hard graft shell composed of styrene and PMMA and a special stabilization formulation. The special stabilization formulation consists of at least one hindered phenol, a phosphite, a sulfide, and optionally a pH buffer system.
In contrast, the addition of common stabilizers to a core shell rubber elastic copolymer either did not improve the heat aging properties to a satisfactory extent or the thermostability in the melt deteriorated to an unacceptable extent.
Polyacetals
A number of methods are known for improving the toughness properties of polyacetals, by incorporating crosslinked or uncrosslinked elastomers, in some cases also grafted elastomers. The following may be mentioned as examples: POM modified with polyurethanes (German Patent No. 1,193,240), POM modified with a graft copolymer prepared on an acrylic ester/butadiene basis (German Patent No. 1,964,156), POM modified with polybutadiene (U.S. Pat. No. 4,424,307) or POM modified with a polydiene/polyalkyleneoxide block copolymer (German Patent No. 2 408 487). However these mixtures do not show sufficient low-temperature impact strength for many applications.
EP 156,285 and EP 181,541 describe mixtures of POM and core shell rubber-elastic graft copolymers in which the particles have been formed from a rubber-elastic core based on polydiene and a hard graft shell with improved low-temperature impact resistance. The aging behavior of these mixtures at elevated temperatures however is not satisfactory, which limits their application for example for automotive parts. The object of the present invention was, therefore, to provide a toughened POM composition which, compared with known systems, possesses, at temperatures down to -40 .degree. C., considerably improved toughness properties, and at temperatures up to 100 .degree. C. good aging properties combined with a satisfactory thermostability in the melt at temperatures up to 230 .degree. C.
It has now been found that this object can be achieved, surprisingly, by employing a specially stabilized toughening component, namely a stabilized core shell graft copolymer composed of a rubber-elastic polybutadiene core, a hard graft shell composed of styrene and PMMA and a special stabilization formulation. The special stabilization formulation consists of at least one hindered phenol, a phosphite, a sulfide, and a pH buffer system.
In contrast, the addition of common stabilizers to a core shell rubber copolymer either did not improve the heat aging properties to a satisfactory extent or the thermostability in the melt deteriorated to an unacceptable extent.